This guide has a few serve & return drills, while this page has a list to various drills for more advanced players. From the list, the defender drills might be of the most interest to you.
This YouTube video provides a drill geared towards improving reaction time on defense, while this 2009 NY Times article describes a drill to improve reaction time when discussing a young player:
As she spoke, Ebuen was filling a silver bowl with about 150 orange
balls to fire at Landers from across the table. The drill is designed
to improve reaction time and muscle memory. Within two minutes, the
bowl was empty, the balls scattered along the floor.
Footwork is also pretty important for returns:
During a lesson, a Chinese coach had advised me not to use my cross
over so much to get the ball, but rather to use simple movements, such
as the side-to-side shuffle, the one-step, and the sideways jump where
the right leg propels the body to the right during a FH stroke, and
then sometimes mix in the crossover when the ball really seems a bit
unreachable. This seems like a very practical idea, and as far as I
have been able to observe, these footwork methods seem to be used very
religiously by Chinese players, which is probably why they tend to be
rather quick on their feet...
There may be products that can help improve reaction time if you're willing to purchase tools - the Newgy Robo-Pong claims to support a variety of drills to improve reaction time for table tennis. (CAVEAT: I've never used this product, so can't determine whether it performs as promised).
Finally, this article suggests that visual acuity is closely associated with good reaction times in many sports - make sure that your vision is good (lasik, glasses, contacts, etc.).
